1. Field of the Invention
The present invention relates to an exhaust secondary air supply system for supplying secondary air to an exhaust passage by using an electric air pump, to decrease harmful components in an exhaust gas from an engine, and more particularly, to an abnormality detecting device for detecting an abnormal state of the electric air pump, an air supply passage, or a flow rate control valve provided in the air supply passage.
2. Description of the Related Art
There is a known technique for detecting an abnormality in an exhaust secondary air supply system by use of an O.sub.2 sensor provided in an exhaust passage. This technique utilizes the knowledge that if the exhaust secondary air supply system is normally functioning, the O.sub.2 sensor is brought into an oxygen-excessive state by supplying secondary air to the exhaust passage, and a signal indicative of a lean state is outputted.
There is also an abnormality detecting device for an exhaust secondary air supply system, which is described in Japanese Patent Application Laid-Open No. 8-61051. This abnormality detecting device is designed to calculate an electric power value from the current and the voltage in an electric air pump, and detect an abnormal state of the electric air pump depending upon whether the power value is in a predetermined range.
In the former technique, the trouble detection cannot be carried out for a period of time until the O.sub.2 sensor is activated after the start of an engine and for this reason, it is necessary to supply secondary air after the activation of the O.sub.2 sensor which occurs after a lapse of a predetermined time from the start of the engine, in order to carry out the trouble detection. However, there is a problem in that the need for the secondary air supply is immediately after the start of the engine, and it is not preferred to carry out the supplying of the secondary air after that, from the viewpoint of the fact that harmful components in an exhaust gas should be decreased.
The latter device suffers from a problem that it is necessary to continuously monitor the power value during operation of the electric air pump and for this reason, the abnormality detection cannot be carried out for a short period of time.